This invention relates to apparatus for tensioning a membrane of sheet material over a support frame, the stretched membrane being used to support a solar reflecting surface. In the manufacture of heliostats that use a taut membrane held over a support frame it is necessary to generate and maintain a uniform tensioning force in the membrane so as to provide a smooth planar surface for high efficiency solar reflection. Problems initially arise with respect to how to pull the membrane to the high tension levels which can be in the 50 to 100 pounds-per-inch range. Conventional methods of installing a tensioned membrane over a support frame required either the membrane alone or the frame alone being loaded during the attachment of membrane to frame. This necessitated the loaded element to be overloaded to achieve the desired final tension. To achieve these extra high loads required investment in a substantial structural fixture. Another area of concern lies with maintaining the stretched membrane at the desirable level of tension. For example, rivets and screws have a significant slippage and will relieve a large portion of the preload, bond materials may creep with time, and welding is difficult since the material is under load. In addition, ambient temperature changes will cause significant tension changes in the membrane, particularly when materials with different coefficients of expansion are used in support frame and membrane. Another problem with the prior art stems from support frame manufacturing errors and the existence of in-plane frame deflections which result in non-uniform membrane strain. In addition, former heliostat designs do not lend themselves to the convenience of attachment of membrane to frame in the field.